On 28.11.2022 09:33, Alexander Asteroth wrote: > >> scheme@(guile-user)> (letrec ((b a)(a 7)) b) >> $1 = 7 > > should be equivalent (of course in a new scope) to: > >> scheme@(guile-user)> (define b #nil) >> scheme@(guile-user)> (define a #nil) >> scheme@(guile-user)> (set! b a) >> scheme@(guile-user)> (set! a 7) >> scheme@(guile-user)> b >> $2 = #nil >
Hi Alex, The only reason the first example returns 7 is because Guile *happens* to bind a to 7 before it binds b to the value of a. The code could have as well returned another value (IIRC Guile uses #<unspecified>) had Guile decided to evaluate the arms of the letrec in a different order. As per the part of the standard you quoted: > the 〈init〉s are evaluated [...] (in some unspecified order) That's what the "unspecified order" in the parentheses is referring to. On the other hand, in your second code example, there's a strict order in which the various expressions will be evaluated. In the moment you type in (set! b a), the value of a has not yet been changed to 7. If you want to use letrec but with a specific order of evaluation of the arms, then you can use the letrec* variant: > (letrec* ((a 7) (b a)) b) > $1 = 7 This will definitely work, without relying on chance or an implementation detail of Guile. -- Taylan
